Looking on the internet deeply has found these results:

She is good at tennis,because he is not is the most popular phrase on the web.

She is good in tennis, because he is not 13 results on the web	She is good at tennis,because he is not 1160,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	She is so good at making people believe she is good with people. The manager is good at dodging responsibility. So she is good at keeping a straight face, and she vigorously crushed any attempt in the House of Lords for the British people to have a referendum. But she is good at her job? Cyril, miraculously, is good at something, and... She is good at playing the guitar. Is she there because she is good at her job, or is she there because she is part of a quota? She is good at speaking English.

She is good in tennis, because he is not

13 results on the web

She is good at tennis,because he is not

1160,000,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

She is so good at making people believe she is good with people.
The manager is good at dodging responsibility.
So she is good at keeping a straight face, and she vigorously crushed any attempt in the House of Lords for the British people to have a referendum.
But she is good at her job?
Cyril, miraculously, is good at something, and...
She is good at playing the guitar.
Is she there because she is good at her job, or is she there because she is part of a quota?
She is good at speaking English.